Chromium phosphate catalyst

ABSTRACT

An amorphous chromium (III) phosphate catalyst is used in alkylating aromatics and in the dehydration of alcohols to ethers.

This application is a divisional application of my co-pendingapplication Ser. No. 578,394, filed Feb. 9, 1984, now U.S. Pat. No.4,543,436.

This invention relates to metal phosphates. In particular it relates tothe use of chromium (III) phosphates as catalysts in the alkylation ofaromatic hydrocarbons. It also relates to the use of chromium (III)phosphates as catalysts in the dehydration of alcohols to ethers. Inparticular, it relates to the use of chromium (III) phosphate as acatalyst used in the methylation of toluene and in the dehydration ofmethanol to dimethyl ether.

BACKGROUND OF THE INVENTION

In order to alkylate aromatic hydrocarbons, it is necessary that analkylating agent be present. Alcohols can serve as alkylating agents.

Therefore, in a feed stream of alcohols and aromatic hydrocarbons, thedehydration reaction of the alcohol occurs at the same time thealkylation reaction occurs. Since xylene, an alkylated aromatic, is apreferred product, the alkylation reaction is the preferred reaction andcatalysts are chosen because of their ability to hinder the dehydrationreaction and advance the alkylation reaction. The dehydration reactionwill occur, however, using the same catalyst.

The alkylation of aromatic hydrocarbons utilizing crystalline catalystsis well known in the art. The alkylation process is used to producexylenes. Xylenes come in three isomeric forms: ortho-, meta- andpara-xylene, the latter of which is of particular value in themanufacture of terephthalic acid which is an intermediate in themanufacture of synthetic fibers such as poly(ethylene terephthalate).

Prior art has shown the use of crystalline aluminosilicate catalysts inthe production of xylenes through aromatic alkylation. One problem withsome of these catalysts is that they tend to lose their activityquickly. These catalysts must also be selective. The reaction must favorparticularly desired compounds, for instance para-xylenes over ortho-ormeta-xylenes. It must also give reasonably high conversion rates.

Therefore, an object of this invention is to provide a catalyst thatwould not lose activity quickly.

Another object of this invention is to provide a selective catalyst. Apreferred object of this invention is to provide a catalyst selective topara-xylene.

Another object of this invention is to provide a catalyst that provideshigh conversion rates.

Another object of this invention is to provide a catalyst that increasesthe production of alkyl-substituted aromatic hydrocarbons.

A specific object is to provide a catalyst for the production of xylene.

Another object of this invention is to provide a catalyst for thedehydration reaction of alcohols.

Other objects will become apparent from the following descriptions.

SUMMARY OF THE INVENTION

In accordance with the present invention, metal phosphates have beendiscovered to be active catalysts for a variety of alkylation anddehydration processes. In particular, chromium (III) phosphates andcombined chromium (III) aluminum phosphates are active for thealkylation of aromatic hydrocarbons and the dehydration of alcohols toethers. In a specific embodiment, it has been discovered that chromium(III) and combined chromium (III) aluminum phosphates are active for themethylation of toluene with methanol and for the dehydration of methanolto dimethyl ether.

DETAILED DESCRIPTION OF THE INVENTION

Chromium (III) phosphates are prepared using conventional methods. Onemethod for producing the chromium (III) phosphates of this inventionincludes precipitating from solutions of dissolved chromium (III) saltsand phosphates or hydrogen phosphates, the solution preferably beingneutral or slightly basic.

The metal phosphate catalyst can possess any ratio of chromium tophosphate. The atomic ratio of chromium to phosphorus will preferably begreater than unity. More preferably, the atomic ratio of chromium tophosphorus will be about 1.2:1 to about 1.7:1.

The metal phosphate catalyst can be present with chromium (III)phosphate alone or in any combination of chromium (III) phosphate andaluminum phosphate. These metal phosphates can be present in anyrelative amounts, from entirely chromium phosphate to almost entirelyaluminum phosphate.

The preferred combined chromium (III) aluminum phosphate can be preparedin any conventional manner. Preferably, the combined phosphate isprepared by coprecipitation of CR⁺³ ions and Al⁺³ ions with PO₄ ⁻³ orHPO₄ ⁻² ions in aqueous solutions.

The metal phosphate catalyst can also be attached to a hydroxide group.The following formula represents metal phosphate catalyst of this group:

    Cr(PO.sub.4).sub.n.(OH).sub.m

wherein n can range from about 0.5 to about 0.9 and m can range fromabout 1.5 to about 0.3.

The alkylation of aromatic hydrocarbons such as benzene or toluene tom-, o-, p-xylene can be carried out with linear or branched alcoholshaving 1 to 10 and preferably 1 to 4 carbon atoms per molecule. Thepresently preferred aromatic hydrocarbon is toluene and the presentlypreferred alcohol is methanol.

The alkylation of an aromatic hydrocarbon, in the presence of the metalphosphate catalyst, is effected by contact of the aromatic hydrocarbonwith an alcohol, at a temperature between about 150° C. and 550° C. andpreferably between about 350° C. and 500° C. The reaction generallytakes place at a pressure from about 50 psig to about 800 psig, butpreferably the pressure will be within the approximate range of 150 psigto about 600 psig.

The molar ratio of the alcohol to the aromatic hydrocarbon is generallybetween about 0.3:1 and about 3:1. When methanol is employed as analkylating agent and toluene is used as the aromatic hydrocarbon, asuitable molar ratio of methanol to toluene has been found to beapproximately 0.4:1 to 1.5:1 moles of methanol per mole of toluene.

The metal phosphate catalyst can be present in any suitable amountnecessary for the alkylation of aromatic hydrocarbons and for thedehydration of alcohol. The reaction is suitably accomplished utilizinga liquid hourly space velocity (volume of feed/hour/volume of catalyst)of between about 1 and about 10 preferably between about 2 and about 7.

The condensed product of the reaction of toluene and methanol comprisingdimethyl ether, p-xylene, o-xylene, and m-xylene can be separated by anysuitable means such as recrystallization at low temperature or selectiveadsorption (e.g., in a chromatographic column). Generally, more o-xyleneis formed than either p-xylene or m-xylene.

The process of this invention may be carried out as a batch-type,semicontinuous, or continuous operation utilizing a fixed or moving bedcatalyst system. The catalyst after use is conducted to a regenerationzone after which the regenerated catalyst is recycled to a conversionzone for further contacting with the toluene and methylating agentreactants. The metal phosphate catalyst can be partially regenerated byair oxidation of about 900° F. for about 16 hours.

The following examples will serve to illustrate the process of thisinvention without limiting the same.

EXAMPLE I Catalyst Preparation

Control catalyst A, Al(PO₄), was prepared by adding a solution of 115 g(1 mol) of (NH₄)H₂ PO₄ in 1 L of water to 237 g (1 mol) of NH₄ Al(SO₄)₂in 1 L of water, giving a clear solution. Precipitation was attempted byurea hydrolysis. Thus, one mole of urea (60 g) was added and thesolution warmed to 60° C. for 48 hours. Precipitation had still notoccurred, so solid (NH₄)₂ CO₃ was added over a 2 hour period toneutralize the solution. The resulting precipitate was collected byfiltration, pulverized and washed with 1 L of dilute NH₄ HCO₃ (10 g/L),then oven-dried and finally calcined at 315° C. (600° F.) for 5 hours.The resulting catalyst had a surface area of 72.3 m² /g and a porevolume of 2.16 ml/g. The catalyst was mostly amorphous, but x-raydiffraction analysis did indicate a small degree of crystallinity.

Invention catalyst B, 1 mole percent Cr on Al(PO₄), was prepared bymixing 1230.5 g (3.3 mol(Al(NO₃)₃.9H₂ O, 227.6 g (2.0 mol) (NH₄)H₂ PO₄,and 15.6 g (0.04 mol) of Cr(NO₃)₃.9H₂ O with 75 mL of water, thenheating the mixture until homogenous. An aliquot of 200 mL of the syrupthus obtained was blended with 90 mL of concentrated NH₄ OH and stirredvigorously. The resulting mixture was heated to about 80° C. and stirredwhile 27.6 g (0.46 mol) of urea were added. After about 45 minutes,gelation occurred. The resulting gel was allowed to age in a forced airoven at 80° C. until liquid phase was no longer observable. The residuewas then washed 3 times with equal volumes of dilute ammoniacal H₂ O,then once with water and finally 2 times with equal volumes of acetone.The washed material was dried overnight in the vacuum oven to give about44 g of clear turquoise gelatinous beads. These were activated for 3hours at 700° C. in an air atmosphere.

Invention catalyst C, Cr(PO₄)₀.7.(OH)₀.9, was prepared by mixing asolution containing 123.5 g (0.5 mol) of Cr(OAc)₃ :H₂ O in about 1 L ofwater with a solution of 39.6 g (0.3 mol) of (NH₄)₂ HPO₄ in about 300 mLof water in a 2 L round bottom flask equipped with a Dean-Start trap.The solution was heated to boiling and about 650 mL of water collected,during which time a blue-green precipitate formed. The pot was cooled,about 450 mL of ethyl acetate was added, and the pot contents againheated to reflux. Azeotroped water was collected at the rate of about 30mL per hour, over about 6 hours. The pot was again cooled, 300 mL ofadditional ethyl acetate was added, and the pot again warmed to refluxto remove remaining water. The majority of the added ethyl acetate wasthen distilled off, then vacuum was applied to aid removal of residualamounts of solvent. The residual blue-green solid was heated to about480° C. in stages under an air flow and held at 480° C. for about 1.5hours. Catalyst turned gray-black when the calcination temperaturereached about 250° C. The resulting material was determined to have asurface area of 132 m² /g and a pore volume of 1.08 ml/g. Elementalanalysis revealed a chromium content of 42.1 weight percent andphosphorus content of 17.6 weight percent. This corresponds to an atomicratio of Cr:P of about 1.4:1.

Invention catalyst D, Cr(PO₄)₀.6.(OH)₁.2, was prepared according to theprocedure described above, except 46.2 g (0.35 mol) of (NH₄)₂ HPO₄ wasemployed. The resulting catalyst is gray, has a surface area of 142 m²/g, a pore volume of 1.15 ml/g and elemental analyses of 37.8 weightpercent chromium and 13.8 weight percent phosphorus. This corresponds toan atomic ratio of Cr:P of about 1.6 to 1.

Invention Catalyst E, Cr(PO₄)₀.6.(OH)₁.2, was prepared by blending asolution of 247 g (1 mol) of Cr(OAc)₃.H₂ O in 300 mL water with asolution of 105.6 g (0.8 mol) of (NH₄)₂ HPO₄ in 400 mL of water. Thesolution was boiled for about 30 minutes, when gelation occurred. Thegel was placed in an evaporating dish and allowed to air dry in the hoodover about 24 hours. The catalyst was further dried for about 8 hours ina forced draft oven at about 105° C. (220° F.). Finally, catalyst wascalcined at about 480° C. for 2 hours. Catalyst was analyzed and foundto have a surface area of 155 m² /g, a pore volume of 0.25 ml/g.Elemental analysis revealed a chromium content of 39 weight percent anda phosphorus content of 13.9 weight percent. This corresponds to anatomic ratio of Cr:P of about 1.6:1.

EXAMPLE II

The various catalysts prepared as described above were tested fordehydration activity and activity for alkylation of toluene. In allcases, 20 mL of catalyst was loaded into the center of a 1/2" diametermetal tubular reactor with a pre-heat and post-reaction zone each packedwith about 6" of glass beads. The reactor was brought up to reactiontemperature under hydrogen pressure, then hydrogen feed discontinued andmethanol or methanol/toluene feed started. Both liquid and gas sampleswere collected and analyzed by gas liquid chromatography. Reactionconditions and results are summarized in the Table.

                                      TABLE                                       __________________________________________________________________________             Feed                                                                 Catalyst,                                                                              mL/hr                                                                             mL/hr                                                                              Reaction Conditions                                                                       Conversion, %                                                                          % Selectivity to                       Run #                                                                             g    MeOH                                                                              Toluene                                                                            Temp, °C.                                                                    Press, psig                                                                         MeOH                                                                              Toluene                                                                            Me.sub.2 O                                                                        Xylenes                            __________________________________________________________________________    1   A, 15.9                                                                            20  115  455   300   >90 <0.5 >90 ˜85.sup.                     2   B, 10.2                                                                            20  115  455   300   100 16   ˜90                                                                         81.sup.1                           3   C,  8.5                                                                            140 --   190   300   1.4 --   ND  --                                 4        120 --   370   300   40  --   ˜90                                                                         --                                 5        40  --   370   300   52  --   >90 --                                 6        40  --   370   300   58  --   >95 --                                 7   E, 21                                                                              30   60  315   150   low low  ND  ND                                 8        20   50  425   150   >90 26   >90 83                                   8A     20  110  540    50   ND  <0.5 ND  ND                                 9   E, 22                                                                              20   90  425   300   ND  21   ND  78.sup.2                           10       30   95  425   600   ND  6.3  ND  87.sup.2                           11       30  100  410   300   ND  2.2  ND  >90.sup.2                          12       30  100  410   300   ND  1.2  ND  >90.sup.2                          __________________________________________________________________________     .sup.1 The composition of the xylenes was: about 29 weight%-p-xylene,         about 20 weight% mxylene and about 51 weight% oxylene.                        .sup.2 The composition of the xylenes was: about 33 weight% pxylenes, 22      weight% mxylenes and 44 weight% oxylenes.                                

The results of these experiments demonstrate that phosphate deficientchromium phosphate catalysts are active dehydration catalysts foralcohols and are active for the alkylation of aromatics such as toluenein the presence of alkylating agents such as methanol.

We claim:
 1. A catalyst composition for the alkylation of aromatichydrocarbons and the dehydration of alcohols comprising a chromium (III)phosphate represented by the formula:

    Cr (PO.sub.4).sub.n . (OH).sub.m

where n ranges from about 0.5 to about 0.9 and m ranges from about 1.5to about 0.3.
 2. A composition according to claim 1 where said chromium(III) phosphate is additionally in combination with aluminum phosphate.